1. Field of the Invention
The present invention relates to a navigation system that shows a current position on a map.
2. Description of Related Art
A navigation system has a plurality of maps, which have different detail rates. The navigation system displays one of the maps by a selection operation of a user to show a current position on the map. The detail rate of the map includes a scale of the map, a displaying density, and so on. The current position is determined by means of a global positioning system (GPS), a map matching, or the like. The GPS uses one of a plurality of measurement methods. In one of the measurement methods, a measurement accuracy of the GPS is relatively low but can be maintained with stability. In another measurement method, the GPS can measure with accurate but with unstable. The measurement accuracy is an accuracy of the determining of the current position.
For example, an autonomous positioning GPS and a differential GPS (D-GPS) belong to the former one. The autonomous positioning GPS has the accuracy of the measurement within the order of ten meters. The autonomous positioning GPS can maintain the measurement accuracy whenever the GPS receives location data from GPS satellites. The D-GPS is a method that determines the current position based on the location data and a correction data. The location data is received from the GPS satellites. The correction data is received from a reference station, which is fixed at a reference position. The D-GPS has the accuracy of the measurement within a few meters. When the D-GPS once receives the correction data from the reference station, the D-GPS can maintain the measurement accuracy for a certain time whenever the D-GPS receives the location data from the GPS satellites even if the D-GPS does not receive the correction data.
A kinematics positioning is an example of the later one, which can measure with accurate but with unstable. The kinematics positioning uses a portion of a carrier wave of the location data, which is transmitted from the GPS satellites, as well as the location data from the GPS satellites. In the kinematics positioning, the measurement of the current position is carried out based on determinations of a path difference and a baseline vector. The path difference is a difference between a first path distance and a second path distance. The first path distance is a distance between the GPS satellites and the reference position. The second path distance is a distance between the GPS satellites and the current position. The baseline vector is a vector of the current position with respect to the reference position when viewed from the locations of the GPS satellites. The measurement accuracy of the kinematics positioning is within a few centimeters, which is accurate than the former one.
In the kinematics positioning, the determination of the baseline vector requires to use a lot of correction data with respect to the reference position. In order to achieve the measurement of the current position in real time in the kinematics positioning, the reference station have to continue to transmit the correction data to a kinematics positioning GPS receiver, which is located at the current position. If the continuous data transmission of a lot of the correction data is not achieved, the kinematics positioning cannot achieve the expected measurement accuracy. Accordingly, in the kinematics positioning, the current position is measured with accurate but with unstable.
By the way, the navigation system has no relation between the measurement method and the detail rate of the map. Accordingly, there is a problem in the navigation system. For example, suppose that a vehicle drives in an urban area and the navigation system mounted on the vehicle shows the current position measured by means of the autonomous positioning GPS receiver on a detail map, such as a city map. In such a situation, since the actual measurement accuracy of the autonomous positioning GPS receiver is the order of ten meters, the current position may be plotted on a distant position in the city map away from an actual current position, such as in a distant road or in a building. This gives an unsuitable feeling to the user.
On the other hand, suppose that the kinematics positioning GPS receiver is used for measuring the current position when the navigation system uses a relatively low detail rate map, such as a wide view of a city, to plot the current position. In such a situation, there is no problem whenever the kinematics positioning GPS receiver measures with accurate. However, the kinematics positioning GPS receiver is unstable as described above, so that the measurement accuracy of the kinematics positioning may be worse than the detail rate of the map, for example in comparison with using the autonomous positioning GPS receiver.
Accordingly, in the vehicle navigation system, when the current position is displayed in the map, the displaying of the map may give the user unsuitable feeling according to a relation between the detail rate of the map and the measurement accuracy.